On 19 March 2009 a BAe 146-200 being operated by South African Airlink on a scheduled passenger flight from George to Cape Town in day Visual Meteorological Conditions (VMC) experienced a flameout of all four engines during the landing roll at Cape Town. The aircraft had enough momentum to roll forward on the runway and vacate onto a taxiway and the APU continued to provide electrical power to the hydraulic system, which facilitated braking and directional control. It was then towed from the taxiway to the apron and the passengers disembarked normally.
Upon becoming aware of the event, which was not reported as a Serious Incident by the aircraft operator, an Investigation was commenced by the Accidents and Incidents Investigations Division of the South African CAA.
It was found that during the landing roll at George immediately prior to the incident flight, the aircraft had experienced a double engine flameout. This had been accompanied by abnormal indications for the Thrust Modulation System (TMS), a limited authority auto throttle system installed as standard on this aircraft type. Line maintenance attention after this occurrence had been unable to find any related fault but extensive engine ground runs had been performed with no recurrence and so the aircraft had been released to service.
During the subsequent (Incident) flight, the aircraft commander, who was PF, observed that the No. 2 engine was not functioning correctly. Later, when on visual downwind to land at Cape Town, the throttles were set to Flight Idle and the No. 2 engine indicated Ground Idle, a speed which should not be selectable in the air. During the landing, abnormal indications were seen on the TMS CDU and, shortly after touchdown with the throttles selected as normal to the Ground Idle position to assist deceleration, all four engines spooled down and flamed out.
The Investigation concentrated on maintenance records which indicated that the TMS had been reported faulty two days earlier and the defect deferred by the responsible aircraft maintenance organisation (AMO) with the TMS deactivated prior to release to service as allowed under the Minimum Equipment List (MEL).
It was noted that, according to the MEL, the TMS may be inoperative provided that three conditions are met:
- All four actuators are centered,
- The system is depowered by pulling and collaring the primary circuit breakers,
- The actuating centering circuit breakers must NOT be pulled.
The Investigation found that:
”the AMO had not complied with the identified MEL conditions in that:
(i) The AMO did not follow the correct maintenance procedure in ensuring that all four actuators were centered, prior to deactivating the system.
(ii) Instead of pulling and collaring only the primary circuit breakers, the AMO also pulled the actuator centering circuit breakers.
(iii) The collaring of the circuit breakers was done with black tie raps.”
Following two flights in this condition, maintenance worked on the aircraft to replace both the TMS Computer and the TMS CDU using components taken from another aircraft. They then reactivated the TMS and cleared the associated Acceptable Deferred Defect However, it was established that only the primary circuit breakers had been reset and the four actuator centering circuit breakers remained tripped. The next day, the aircraft flew six scheduled flights with no TMS defects reported. The first flight of the following day was the one immediately before the Investigated Incident,
After the Incident flight, the TMS was again deactivated and the aircraft released to service again under MEL conditions with the four tripped actuator circuit breakers only being found later.
The Investigation concluded that all four engines spooled down and flamed out uncommanded because the Thrust Modulation System (TMS) was malfunctioning and that the cause of this was that the aircraft maintenance organisation involved had failed to comply with applicable MEL procedures when deactivating / reactivating the TMS. More generally, it was concluded that Aircraft Maintenance Manual procedures for TMS maintenance had not been followed and that there had been no record of the tripping of the four centering actuator circuit breakers in any maintenance documentation. As a result, line station maintenance had been unaware that this action had been taken.
Three Safety Recommendations were made to the Commissioner for Civil Aviation as a result of the Investigation that they should instruct the South African CAA Operations and Airworthiness Departments to:
- Ensure that the M&O MEL procedures do not refer to a document that is not carried on board. The MEL and Ops manual should be amended to clearly reflect the operating procedures in the event of TMS or other system malfunctions.
- Ensure that the operator requires that all flight crew and AMO personnel receive additional training in dealing with TMS and other system malfunctions.
- Enhance their oversight mandate with regards to the operations of the AMO, so as to verify that all procedures and regulatory requirements are adhered to.
The Final Report of the Investigation Serious Incident Report CA18/3/2/0707